1. Field of the Invention
The present invention relates to an image recording apparatus. More specifically, the invention relates to the structure that supports the shaft portion of a conveying roller.
2. Related Background Art
As one example of the conventional image recording apparatus, there is the so-called serial recording apparatus that performs recording on the surface of a sheet by moving the carriage, which mounts a recording head thereon, in the direction orthogonal to the sheet conveying direction (hereinafter referred to as the sheet widthwise direction). Then, the structure is arranged to provide the serial recording apparatus of the kind with a sheet-conveying device that conveys the sheet under the carriage, and record images on the sheet, while repeating alternately the sheet conveyance by the sheet conveying device and the recording by use of the carriage.
Now, in recent years, it has been increasingly demanded for the reasons that images should be formed in higher image quality that the pixel pitches of a recording head is made smaller, while enhancing the conveyance precision of a sheet with respect to the sheet-conveying device.
Here, for the conventional sheet-conveying device, it has been practiced to provide a conveying roller as sheet conveying means, while supporting the conveying roller with the cylindrical bearing portion having a diameter slightly larger than the diameter of the shaft portion arranged for the apparatus main body.
Then, with the structure thus arranged, the shaft portion of the conveying roller and the bearing are in contact only on one contact line. For example, when the conveying roller is at rest, the contact line of the shaft portion of the conveying roller and the bearing is positioned on a designated position by means of the pressurized direction of the pinch roller that is arranged to press the sheet to the conveying roller, the pressure exerted by the conveying roller and the weight of its own, among some others.
On the other hand, when the conveying roller rotates or it is in transition from the stationary condition to the rotation or the like, the rotational acceleration, frictional coefficient, and various other conditions are added, and although the same when it is at rest, the position of the contact line is caused to change minutely due to the delicate configuration of the shaft portion of the conveying roller or the bearing, such as the degree of roundness thereof. Then, such change of the positions of the contact line means the delicate changes of the position of the conveying roller, and such delicate positional changes of the conveying roller presents a problem in materializing the conveyance precision that should be made more precise.
Also, if the friction load between the shaft portion of the conveying roller and the bearing is not stable, the variation of stationary position tends to occur when transition is made from the rotation to the stationary condition. Further, if the friction load is larger, the rotational load of the conveying roller increases to hinder the intended high-speed operation.
Furthermore, there is a need for the provision of an installation space for the bearing on the entire circumference of the conveying roller in the vicinity of the shaft portion of the conveying roller. Consequently, a problem is encountered that the position of the bearing or the like should be restricted.
For example, if the conveying roller the strength of which is weak should be supported only at both ends thereof, the conveying roller warps in the central portion to make it impossible to maintain the distance between the carriage and the sheet constantly or to invite the deterioration of conveyance precision of the conveying roller, hence producing unfavorable effects on the precision of the recorded images. Therefore, it should be considered to arrange a bearing on the central portion of the conveying roller in order to prevent the warping of the central portion of such bearing.
However, if a bearing is arranged on the central portion of such conveying rollers there occurs a need, as described earlier, for the provision of the installation space on the entire circumference of the conveying roller for the bearing in the vicinity of the shaft portion. Then, a problem is encountered that the conveyance of the sheet is impeded by the presence of such bearing after all.
On the other hand, it is attempted to enhance the precision of components of the conveying roller as another method to comply with the request for the enhancement of the conveyance precision of the sheet. Here, it is known that if the eccentric precision, which indicates the fluctuation of rotational center of the conveying roller, is improved in particular, the conveyance precision of the sheet is effectively enhanced.
Therefore, there has been proposed a conveying roller such as the one provided with a thin film coating or the like on the circumference of a metallic shaft that forms the shaft portion in order to generate conveying force, or the so-called metallic roller that generates the conveying force with the irregularities given to the surface of the metallic shaft, among some others. Since each roller of these kinds is formed to provide substantially the same diameters for the shaft portion serving as the rotational center and the conveying portion that has the conveying force, the fluctuation of the rotational center is made considerably smaller with respect to the conveying portion.
Nevertheless, even for such a metallic roller as this, the degree of straightness of the conveying roller or the like exerts a great influence on the conveying portion away from the bearing, for example, there is a problem encountered that the fluctuation of rotational center becomes great, although there is almost no fluctuation of the rotational center on the conveying portion near the bearing.
On the other hand, it is also required to increase the pressurized contact of the pinch roller in order to press the sheet to be in contact with the conveying roller for the enhancement of the conveyance precision of the sheet. Along with this, there is a demand in making the apparatus smaller at lower costs, which necessitates the provision of the structure of a conveying roller made smaller yet allowing no deformation even with a designated pressure.
It is an object of the present invention to provide an image forming apparatus for which the enhancement of sheet conveyance precision is possible or the conveying roller can be made smaller, and the reduction of costs is possible as well.
It is another object of the invention to provide an image forming apparatus provided with a conveying roller for conveying a sheet, and a bearing portion fixed to the apparatus main body for rotatably supporting the conveying roller through the shaft portion of the conveying roller, in which the bearing portion has at least two contact portions to be in contact with the shaft portion of the conveying roller, and the shaft portion of the conveying roller is biased to the contact portions.